One blistering afternoon in Iraq, while fighting insurgents in the northern town of Mosul, Sgt. Sinque Swales opened fire with his .50-cal. That was only the second time, he says, that he ever shot an enemy. A human enemy.

"It felt like I was in a big video game. It didn't even faze me, shooting back. It was just natural instinct. Boom! Boom! Boom! Boom! " remembers Swales, a fast-talking, deep-voiced, barrel-chested 29-year-old from Chesterfield, Va. He was a combat engineer in Iraq for nearly a year.

Like many soldiers in the 276th Engineer Battalion, whose PlayStations and Xboxes crowded the trailers that served as their barracks, he played games during his downtime. "Halo 2," the sequel to the best-selling first-person shooter game, was a favorite. So was "Full Spectrum Warrior," a military-themed title developed with help from the U.S. Army.

"The insurgents were firing from the other side of the bridge. . . . We called in a helicopter for an airstrike. . . . I couldn't believe I was seeing this. It was like 'Halo.' It didn't even seem real, but it was real."

This is the video game generation of soldiers. " 'Ctrl+Alt+Del,' " the U.S. Army noted in a recent study, "is as basic as 'ABC.' " And computer simulations -- as military officials prefer to call them -- have transformed the way the United States military fights wars, as well as soldiers' ways of killing.

"There's been a huge change in the way we prepare for war, and the soldiers we're training now are the children of the digital age who grew up with GameBoys," says retired Rear Adm. Fred Lewis, a 33-year U.S. Navy veteran who now heads the National Training Systems Association, a trade group that every year puts on the Interservice/Industry Training, Simulation and Education Conference, the military counterpart of the glitzy Electronic Entertainment Expo. "Live training on the field is still done, of course," but, he adds, "using simulations to train them is not only natural, it's necessary."

War is no game, of course, but games, in a big way, have updated war. The weapons Swales uses when he plays "SOCOM 3: U.S. Navy SEALS," for example, are virtual replicas of the weapons he used as a soldier in Iraq.

"The technology in games has facilitated a revolution in the art of warfare," says David Bartlett, the former chief of operations at the Defense Modeling and Simulation Office, a high-level office within the Defense Department and the focal point for computer-generated training at the Pentagon. "When the time came for him" -- meaning Swales -- "to fire his weapon, he was ready to do that. And capable of doing that. His experience leading up to that time, through on-the-ground training and playing 'Halo' and whatever else, enabled him to execute. His situation awareness was up. He knew what he had to do. He had done it before -- or something like it up to that point."

In the mid-1990s, Bartlett, an avid gamer himself, created "Marine Doom," the military version of the original "Doom," the granddaddy of first-person shooter games. The simulation was conducted in a lab with six PCs networked together. It served as a precursor for more expensive, highly immersive, state-of-the-art military simulation centers and PC labs. Some, like the Asymmetric Warfare -- Virtual Training Technology, largely train soldiers how to coordinate complicated missions. Think of it as a sort of military "EverQuest" that can be played by multiple people in multiple places at the same time. With the Indoor Simulated Marksmanship Trainer, soldiers train to effectively shoot their weapons by holding a rifle that looks like an M16, except it fires a laser and the target is a giant screen.

Lt. Col. Scott Sutton, director of the technology division at Quantico Marine Base, where the mock-up M16s are used, says soldiers in this generation "probably feel less inhibited, down in their primal level, pointing their weapons at somebody." That, in effect, "provides a better foundation for us to work with," he adds.

No one knows for sure whether Sutton is right. Since at least World War II, studies purporting to explore how readily troops pulled the trigger -- S.L.A. Marshall's "Men Against Fire," for example -- have aroused controversy and been scored as anecdotal. Indeed, collecting data in the swirl of battle is no less formidable a challenge today than in the past. As a result, comparisons to previous generations of soldiers are problematic. Nonetheless, soldiers today are far more knowledgeable about weaponry than their predecessors, Bartlett feels sure, and have "a basic skills set as to how to use them."

Retired Marine Col. Gary W. Anderson, former chief of staff of the Marine Corps Warfighting Lab, agrees. And he takes it a step further: Today's soldiers, having grown up with first-person shooter games long before they joined the military, are the new Spartans, he says.